In optical transmission systems and especially in systems having long unrepeatered fiber spans, it is important to launch as high an optical power into the transmission fiber as possible. Unfortunately, the amount of launch power at a particular wavelength is limited by nonlinear phenomena such as stimulated Brillouin scattering ("SBS"), stimulated Raman scattering, self-phase modulation, four-photon mixing and cross-phase modulation. These phenomena can degrade the optical signals and increase bit error rates for the data.
Narrow linewidth laser sources are particularly vulnerable to SBS problems because the SBS power threshold increases with increasing linewidth of the optical signal being propagated. The SBS power threshold is arbitrarily defined as the input optical pump signal power level at which the powers of the input optical pump signal and the backward Stokes signal become equal. It is well known that dithering of the source optical wavelength for a narrow linewidth source is a useful technique for overcoming the power limitations imposed by SBS. Specifically, the source wavelength can be swept over some narrow band of wavelengths by directly modulating the source laser with a periodically varying dithering current. This technique permits higher launch power to be employed--well above the original SBS threshold for the source without dithering--without incurring the penalty of SBS. Present experiments have shown that a dither rate, for the source wavelength, at or above approximately 5 kHz is effective for suppressing SBS. See, for example, U.S. Pat. No. 5,329,396 issued Jul. 12, 1994.
But, there is one problem for this dither technique. As the dither current sweeps back and forth, it imparts an amplitude modulation at the dither rate. The amplitude modulation varies the envelope of modulated signal. For intensity modulated data, this additional or residual amplitude modulation on the intensity modulated signal penalizes the optical receiver by causing a closure of the eye pattern and thereby increasing the difficulty of discriminating between zero and one optical data bits. In other words, the error rate increases with increasing dither rate. To date, no one is known to have addressed or solved this problem.